The present invention relates broadly to a fastener drive system and cooperable driving tool. More specifically, the present invention relates to a fastener drive system for applications where the presence of fluid, such as in the attachment of a medical prosthesis to bone, in a very small clearance gap between the driver and the fastener may result in the generation of hydraulic forces inhibiting free movement of the driver with respect to the fastener.
Medical prosthetic arts have advanced substantially in both design and the materials used to create prosthetics. Typically, a combination of mechanical fasteners and adhesives are used to secure a prosthesis to surrounding bone. A high degree of precision is required in order to securely retain the prosthesis in a proper working position. When the prosthesis is fitted to surrounding bone it is preferred to make the prosthesis fit correctly the first time to avoid having to make adjustments. Correct placement of the fastener is particularly important in craniomaxillofacial surgeries where a very small fastener must be placed in a precise location.
The treatment of fractures of the craniomaxillofacial region proceeds by reducing the fractured bones to their anatomically correct positions, and there fixing the bones in place. One common way to fix the bone fragments in place is by the technique of mini-plate fixation. The mini-plate is held in place by a series of small bone screws extending through the plate and into the bone. Typically, these screws are very small with an outer thread diameter ranging from 1.0 mm to 5.0 mm.
In many surgical applications, the placement of the screw is limited to a precise location either by the anatomical structure or because of the size of the fragment to be fixed. Moreover, the bone at the site of fixation is often relatively thin, particularly in children, thus increasing the risk of screw pull out. Should fluid enter the recess of either the driver or the fastener, there is a potential for the bone screw to strip the bone holding the screw as a result of hydraulic forces generated by the trapped fluid which acts to hold the driver within the socket.
In the field of orthopedics, fasteners and drivers of various types have long been employed to transmit rotational force from the driver to the fastener. One of the simplest fastener-driver combination is a slotted recess in the fastener head. Other drive forms include a slot with a center alignment hole, crucifrom, phillips, square, hexagon, and hexalobe. Each of these designs offers desirable features either in ease of manufacture or reliable torque transmission. For each of these drive systems, the geometric configuration utilized by the fastener drive area is repeated in the drive end of the driver with only slightly different dimensions to allow sufficient clearance to be slideably received therein.
In an effort to increase torque transmission and limit camming out during the torque transmission procedure, the tolerances between the driver end and the fastener driving area have continued to be reduced. Moreover, as disclosed in European Patent Publication 0,458,449 in some applications the drive end includes a spacer element to ensure secure engagement of the driver with the socket. Still further, U.S. Pat. No. 4,269,246 discloses that it is desirable that the driver head have a taper to encourage all of the surfaces of the driver to securely engage the surfaces of the socket, thereby increasing the frictional fit between the socket and driver.
As the need for smaller screws and other miniature fasteners have developed, particularly in the medical field, so to have the manufacturing tolerances necessary to provide a secure match between the driving tool and the fasteners recessed driving socket. While the tolerances utilized in small screws, i.e. often with a thread diameter less than 5 mm, has been effective to provide the desired torque transmission capacity, another relatively unexpected problem has developed.
Very tight tolerances required to manufacture small fasteners provide very little clearance between the driver and the drive area of the fastener. Any fluid trapped in these tight areas may generate hydraulic forces which can inhibit movement of the driver within the drive area. The probability that fluid will enter the driving engagement is increased when used in medical procedures were bodily fluids, irrigation fluids, and other fluids are routinely present. This creates a problem because the driving tool may not be fully seated in the socket and therefore may have a tendency to cam out of the socket or to allow stripping of the fastener driving area and/or dulling of the driving tool. This problem is intensified by having very small screws and driving tools were the amount of metal to resist the torque exerted when inserting the screw is extremely small. Thus there is a greater potential for stripping of the socket or dulling of the driving tool if the tool is not properly seated to take advantage of the entire driving surfaces of the socket.
Another problem related to the short coming discussed above, is that even if enough force can be applied to overcome the hydraulic forces and properly seat the driver in the socket, the driver may be locked into the socket by hydraulic forces resulting from the fluid held between the driver and the socket. Essentially, the surface tension of any fluid at the bottom of the socket and disposed between the driving tool and walls of the socket hydraulically holds the driving tool in place. Because of the close tolerances between the driving tool and socket side walls the fluid cannot readily move to allow air or some other fluid to enter and relieve the hydraulic lock. As a result an exceptional amount of axial force must be applied to the driver to remove it from the fastener.
In the case of small screws, particularly those placed in relatively soft material (i.e. bone), the force applied to remove the driver can over come the purchase of the screw in the receiving material. This is especially troublesome in medical procedures where there may only be a single location for a bone screw to be placed. In these instances, pull out of the original bone screw strips the hole and may require the use of bone cement or a larger diameter screw to be used.
A need therefore exists for a fastener driving socket and cooperable driving tool which provide a channel for the passage of fluid upon engagement and disengagement of the driver with the fastener. Such a driving socket and cooperable driving tool must also provide effective torque transmission.